1. Field of the Invention
The present invention generally relates to a cam timing control device for use with an automotive internal combustion engine and, more particularly, to the cam timing control device of a type wherein an electromagnetic force is utilized to change the cam timing to secure a high response to a switching signal used to change the cam timing.
It is to be noted that the term "cam timing" herein used is to be understood as meaning an operational timing of the cam, drivingly associated with an automotive fuel intake valve, relative to the selective opening and closure of such fuel intake valve.
2. Description of the Prior Art
In an automotive engine system, for selectively opening and closing the intake and exhaust valves, it is a general practice to drive the camshaft synchronously with the engine by the utilization of a drive produced in the engine. In this system, it has been suggested to change the cam timing in dependence on the engine operating condition, for example, one or both of the load imposed on the engine and the engine speed, so that a valve timing between the intake valve and the exhaust valve can be adjusted to increase the operational efficiency of the engine as a whole.
An example of conventional cam timing control devices used for changing the cam timing is known to be of a hydraulic type wherein a hydraulic mechanism is utilized.
Also, the U.S. Pat. No. 4,841,924, issued Jun. 27, 1989, discloses another example of the conventional cam timing control devices, which is so designed as to change the cam timing electrically.
More specifically, the above mentioned patent discloses an electrically actuated cam timing control device for varying the angular phase relation between the camshaft and the crankshaft. This known cam timing control device comprises an engine timing pulley mounted on the camshaft for rotation coaxially of the camshaft over a predetermined angle and a biasing spring interposed between the timing pulley and the camshaft for biasing the camshaft in a retarding direction by means of a coil spring. The known cam timing control device also comprises a clutch mechanism adapted to be actuated, when an electric power is supplied from a battery to an electromagnetic coil, to produce a frictional force necessary to advance the camshaft when balanced with the biasing force of the coil spring.
The electrically actuated cam timing control device of the type referred to above is so designed that the balance between the frictional force generated in the clutch mechanism and the biasing force of the coil spring is utilized to change the angular phase relationship between the crankshaft and the camshaft, that is, the cam timing. Accordingly, the following problems have been found inherent in this known cam timing control device.
Specifically, during a particular engine operating condition, such as, for example, a high load engine operating condition or a high speed engine operating condition, in which a resistance is relatively high due to a rotational inertia force, the camshaft tends to be halted at a position generally intermediate between predetermined advanced and retarded positions relative to the timing pulley with the response to the switching consequently lowered. Also, when the voltage of an electric power source, for example, the battery, is low and/or the engine temperature is high, a resistance on the side of the electromagnetic coil is high and, therefore, the response to the switching tends to be lowered.
In addition, since an electric power from the battery is merely supplied to the electromagnetic coil, not only does the electrically actuated cam timing control device of the type referred to above tend to consume a relatively large amount of the electric power, but also the alternator generally driven by the engine tends to be loaded. In particular, where the load on the alternator is high, the engine is correspondingly loaded, resulting in a reduction in mileage.
Furthermore, the structure wherein the frictional force generated in the clutch mechanism then actuated is balanced with the biasing force of the coil spring to advance the camshaft has a problem in that the frictional force of the clutch mechanism tends to increase the internal resistance in the engine to such an extent as to result in a reduction in mileage and also to result in a reduction in engine performance such as a resistance to vibrations during an idling operation and engine troubles.
In order to secure the favorable engine performance and the favorable mileage, it may be contemplated to lower the voltage to be supplied to the electromagnetic coil. However, this method would result in a problem in that the length of time during which the change of the cam timing is taking place during a normal engine operating condition will be prolonged. Once this occurs, a proper cam timing cannot be secured and, hence, the engine will not provide a desired output drive, resulting in a reduction in drivability of an automotive vehicle.